The present invention relates to an antenna level display device, an antenna level display method and a receiving apparatus adapted for displaying an antenna level in a digital satellite broadcast receiving tuner or a television receiver having such a tuner therein.
A satellite broadcast receiving tuner or a satellite broadcast receiving apparatus such as a television receiver with an internal tuner is equipped with an antenna level display function for displaying the CN (carrier-to-noise) ratio of an input signal received from a satellite by an antenna. This antenna level display function is utilized to fulfill two broadly classified roles.
The first role is to realize proper directional adjustment of an antenna at the time of installing the antenna and adjusting its direction, in such a manner that the level of the received signal obtained from the antenna becomes maximum. In satellite broadcasting, a parabolic antenna having sharp directivity is used to receive the signal from the satellite. The CN ratio of the received signal is set to a maximum attainable value by properly adjusting the antenna direction while watching the displayed antenna level, so that a wide margin of the CN ratio can be ensured until the reception becomes impossible, hence achieving a satisfactory receiving environment suited to deal with deterioration of the environment due to rainfall or the like.
And the second role is to search for the cause of any disturbance in the reception. Generally, occurrence of a reception disturbance is derived from deterioration of the receiving environment or from some fault in the receiver. It is possible to judge, from the antenna level being displayed, whether the reception disturbance is derived from any environmental deterioration or not. That is, in case the reception disturbance has occurred due to the environmental deterioration, a search is made as to whether the CN ratio of the received signal is sufficiently high or not. And if the antenna level is not sufficient, the disturbance can be judged to result from some environmental deterioration in the reception. In this case, the cause may be such that the antenna is not directed exactly toward the satellite, or there exists a building or the like as an obstacle to the broadcast waves. Also in this case, the cause may include a fault of a converter, breaking of a feeder, failure of a connector and so forth. In contrast therewith, if the antenna level is sufficient, the condition can be so judged that at least the signal from the satellite is properly received by the antenna and that the tuner or the television receiver is probably faulty.
In the conventional analog satellite broadcast receiver, an antenna level is displayed in accordance with the gain of an AGC (Automatic Gain Control) amplifier (as disclosed in, e.g., U.S. Pat. No. 3,134,412). More specifically, an intermediate frequency amplifying stage in the satellite broadcast receiver has an AGC circuit for keeping the level of a received signal constant. In the AGC circuit, the level of the received signal is detected, and then the gain of the AGC amplifier is set in accordance with the detected level of the received signal. And the set gain of the AGC amplifier is displayed as an antenna level.
Thus, in the conventional analog satellite broadcast receiver, the antenna level is displayed on the basis of the gain of the AGC amplifier. However, in a digital satellite broadcast receiver where digital modulation is adopted, a sufficiently high precision is not attainable by displaying the antenna level in accordance with the gain of the AGC amplifier. For this reason, in the known digital satellite broadcast receiver, a CN ratio is calculated from a constellation representing the coordinates of mapped signal points on an IQ plane.
That is, in digital satellite broadcast, 8PSK (Phase Shift Keying) for example is adopted as a modulation mode. In this 8PSK modulation, data are arrayed correspondingly to eight signal points P1 to P8 on an IQ plane having an I-axis and a Q-axis orthogonal thereto, as shown in FIG. 7.
Assuming generally that the entire noise included in the signal waves is composed merely of random noise, the points of the received signal are dispersed, as shown in FIG. 8, around signal points S1 and S2 which are obtained in an original noiseless state, in a probability distribution conforming with a normal distribution. More specifically, the point of the received signal to be positioned at a signal point S1 is dispersed as indicated by a curve A1 in FIG. 8, and also the point of the received signal to be positioned at an adjacent signal point S2 is dispersed as indicated by a curve A2. Due to such dispersion, the received signal within an area L1 beyond an intermediate point between the signal points S1 and S2 of mutually adjacent codes is received as a wrong code.
On the basis of such relationship, the deviation of the received signal point and the CN ratio can be correlated with each other. That is, if the noise included in the signal waves is assumed to be random noise, the deflection of the received signal point on the IQ plane corresponds to the CN ratio.
In calculating the CN ratio from the mean value of the deflections of the received signal points on the IQ plane, the received signal points are mapped on the IQ plane by a demodulator, and then the mean value of the deflections of the individual signal points is measured from the I-signal and the Q-signal. And a measuring system is prepared for setting a modulation signal of a desired CN ratio by superposing random noise. In this measuring system, there is measured the mean value of the deflections of the received signal points. Then a conversion table is formed with regard to the mean value of the measured deflections of the individual signal points and the CN ratio. And the conversion table is stored in a ROM (Read Only Memory).
Upon reception of the input signal, points of the received signal are mapped on the IQ plane by the demodulator, and the mean value of the deflections of the signal points is measured. Then the CN ratio is calculated from the mean value of the deflections of the received signal points by using the conversion table stored in the ROM. And the CN ratio thus obtained from the deflections of the received signal points mapped on the IQ plane is displayed as an antenna level.
In the Japanese digital satellite broadcast, the stationary satellite located above the equator and at 110° C. of east longitude is utilized through a network of 12 GHz band, as in the precedent analog satellite broadcast. Therefore, in shifting from the analog satellite broadcast to the digital one, the existing antenna used previously for the analog satellite broadcast is still usable. In shifting from the analog satellite broadcast to the digital one, if the existing antenna for the analog satellite broadcast is used continuously, it is not necessary to purpose a new antenna or to readjust the direction of the antenna either, hence realizing a ready shift from the analog satellite broadcast to the digital one. For this reason, when receiving the digital satellite broadcast, many users continuously use the existing antenna which has been used for receiving the analog satellite broadcast.
However, there is reported a problem that, when receiving the digital satellite broadcast by the antenna used previously for the analog satellite broadcast, a reception disturbance occurs though the displayed antenna level indicates a sufficient CN ratio. The cause of such a problem is considered as follows.
An antenna for receiving a satellite broadcast is equipped internally with a converter for converting a received signal of 12 GHz band into an intermediate frequency signal of 1 GHz band. Some of the known converters in the antennas for receiving the analog satellite broadcast contain much phase noise. However, since the analog satellite broadcast is transmitted through frequency modulation of analog video signal, high tolerance is ensured against the residual FM noise and therefore proper reception can still be achieved even by an antenna equipped with a converter containing much phase noise.
Meanwhile in 8PSK modulation employed for the digital satellite broadcast, the distance between signal points is short and, when the phase noise is great, it is erroneously judged to be an adjacent code by the demodulator, so that the reception characteristic may be deteriorated. Therefore, if the digital satellite broadcast is received by the antenna used originally for receiving the analog satellite broadcast, a reception disturbance may be caused by the phase noise in the converter.
When such a reception disturbance has occurred due to some harmful influence of the phase noise in the converter, an adequate measure may be taken with facility if the antenna level being displayed reflects faithfully the deterioration of the CN ratio derived from the phase noise.
That is, as mentioned, one role of displaying the antenna level is to search for the cause of a reception disturbance upon occurrence of such disturbance. Therefore, the antenna level is confirmed when a reception disturbance has occurred due to some harmful influence of the phase noise in the converter for example. At this time the carrier level to the noise is lowered, so that if the deterioration of the CN ratio derived from the phase noise is faithfully reflected, the CN ratio being displayed as an antenna level is also lowered. And when the antenna level is lowered, it is seen that the cause is not concerned with at least the satellite broadcast receiver or the television receiver and that the cause is derived from deterioration of the receiving environment of the antenna system, whereby an adequate measure may be taken with facility.
However, in the conventional digital satellite broadcast receiver where the noise is assumed to be random noise as mentioned, the CN ratio is calculated from the deflections of the signal points on the IQ plane, so that the deterioration of the CN ratio due to the phase noise fails to be faithfully reflected.
More specifically, in the known method of measuring the CN ratio in the digital satellite broadcast, the noise is assumed to be random noise conforming with a normal distribution, but the noise superposed on the actual received signal is not limited to random noise alone and may include phase noise therein as described. The phase noise partially includes deviations of frequency components.
If the noise is random one conforming with a normal distribution, the points of the received signal are distributed in the shape of a true circle, as shown in FIG. 9A. However, if some phase noise is included, the points of the received signal are not distributed in the shape of a true circuit and are dispersed in the circumferential direction, as shown in FIG. 9B (Reference paper: IEEE Trans. On Consumer Electronics, Vol. 41, No. 3, August 1995, “QAM FOR TERRESTRIAL AND CABLE TRANSMISSION”). Therefore, if any phase noise is included in the received signal, exact evaluation of the CN ratio is not attainable according to the known CN ratio conversion table prepared on an assumption that the points of the received signal are distributed in the shape of a true circle.
Thus, in the conventional display of an antenna level in the digital satellite broadcast, the CN ratio fails to be faithfully measured with regard to the phase noise. And therefore, when a reception disturbance has occurred due to the phase noise in the converter for example, the measured CN ratio may erroneously be displayed as a sufficient antenna level. Consequently, the expense burden on both consumers and manufacturers for search, inspection and so forth is rendered great.
There may be contrived a method of adding frequency deviation noise as a new item of phase noise and displaying the same. However, if the random noise and the phase noise are both displayed, the general user is obliged to comprehend the information of such two noises and to deal with them separately, whereby the burden is further increased.
There is known a method of indicating the antenna level by counting the number of actual errors as an index to faithfully represent the harmful influence exerted on the picture quality by the deterioration of the received signal. However, if this method of indicating the antenna level by counting the number of errors is adopted, there arises another problem that the time required for measuring the CN ratio becomes longer with a rise of the CN ratio.
FIG. 10 shows bit error rates in relation to respective CN ratios and the time required for observing one error in each case. As shown in FIG. 10, the bit error rate becomes lower with a rise of the CN ratio to thereby prolong the time required for observing one error. For example, when the CN ratio is 6 dB, the time required for observing one error is 1.2 msec. However, when the CN ratio rises to 12 dB, the time required for observing one error becomes 460 sec. Therefore, if the method to indicate the antenna level by counting the number of errors is adopted, a longer time is required in measurement particulaly when the CN ratio is higher.
Thus, in the method to indicate the antenna level by counting the number of errors, longer-time measurement is necessary with a rise of the CN ratio, so that it becomes difficult to adjust the antenna position while watching the displayed antenna level. More specifically, in initial setting, the user adjusts the antenna direction in a manner to maximize the antenna level while watching the displayed antenna level. In this case, it is desired that the time required for attaining a reflection of the CN ratio in the antenna level should be within a range of 0.5 to 1 second. Further, when the antenna position is to be adjusted, reduction of the CN ratio due to attenuation by rainfall or the like needs to be taken into consideration in the digital satellite broadcast, so that it is important to set the antenna level as high as possible at the time of adjusting the antenna level, and the CN ratio needs to be given a wide margin to an unreceivable state. However, in the system of measuring the CN ratio from the bit error rate, it is impossible to achieve proper measurement of the CN ratio in a short time particularly when the CN ratio is high, whereby proper adjustment of the antenna direction is not attainable while watching the indicated antenna level.